Residual symptoms such as tiredness, muscle weakness and muscle pain are observed in patients treated with antidepressants. It is not yet known if some of the residual symptoms of depression, and the negative effects on the skeleton may be mediated via decreased levels of vitamin D. Any decrease in vitamin D may be mediated either by the antidepressants or due to the depression per se (decreased exposure to sunlight and poor dietary habits). The treatment with antidepressants may improve the depression, but may not correct the vitamin D deficiency leaving residual symptoms, which may be interpreted as depression-related and this as incomplete response to the antidepressant treatment. Treatment with vitamin D may thus help alleviate some of the symptoms otherwise attributed to the depression leading to treatment with antidepressants (confounding by indication). Alternatively, antidepressants may interfere with biological processes that affect uptake or metabolism of Vitamin D resulting in low levels of the vitamin D that may have detrimental effects on bone turnover and muscle function.